The present invention relates generally to a new and improved universal chuck for a machine for piercing a taphole of a shaft furnace. More particularly, this invention relates to an improved universal chuck capable of both transmitting a tensile or pulling force to the end of a piercing rod and a rotational moment or force to a drill bit.
It is known that the taphole of a shaft furnace can be pierced either by normal drilling or by the lost rod method.
In normal drilling, a drill bit is driven in rotation by a working member mounted on a mount which is aligned with the axis of the tap hole. This method consequently employs a drill bit, which cuts in rotation and which is coupled to the spindle of the working member in order to make the tap hole. The drill bit is usually equipped with an axial channel which traverses it longitudinally and which enables pressurized air to be conveyed to the tip of the drill so as to effect a better removal of the waste fragments from the drilling and also cool the tip of the drill bit. The device may be a chuck which is fairly simple, and of light weight construction screwed to the spindle of the working member, which enables a rotational moment to be transmitted to the drill bit.
In the lost rod method, after the tap hole has been sealed with a plugging compound, but before the taphole clay has completely hardened, a metal rod is inserted into the tap hole. When the tap hole needs to be opened, the rod is extracted to form an opening in the hardened plugging compound. To extract the rod from the tap hole, it is known to equip the working member of a piercing machine with a special coupling device for firmly joining the free end of the rod to the working member. An axial tensile force is thus transmitted to the rod usually via the blows of a hammer which forms an integral part of the working member.
Such special coupling devices are disclosed, for example in Luxembourg Patent LU-83,917, filed on Feb. 3, 1983 and Luxembourg Patent LU-87,546, filed on Jun. 30, 1989 (corresponding to U.S. Pat. No. 5,056,968), both of which are assigned to the assignee hereof, all of the contents of which are incorporated herein by reference. The two documents provide clamps which can be screwed to the threaded spindle of the working member. They comprise a body equipped with a front bore intended to receive the free end of the piercing rod and two movable jaws which are arranged symmetrically about this front bore and which can be displaced under the action of pneumatic jacks to grip the said free end.
These clamps are, however, not designed to transmit a rotational moment to a drill bit. Indeed, the rotating of the clamp to transmit a substantial moment to a drill bit held between the jaws would inevitably damage the clamp. It must also be remembered that the aforementioned clamp is supported at only one of its ends by the spindle of the working member, and that the clamp weighs approximately 50 kg. In other words, the clamp is much heavier than the chuck generally used to drive the drill bit. Therefore, it would appear to be impossible to rotate the clamp at 150 revolutions per minute to drive a drill bit.
It was also noted that the clamp was often subjected to forces which were offset relative to the axis of the spindle when the lost rod method was applied. These offset forces induce unacceptable bending moments in the spindle and in the mechanism of the working member.
To overcome this problem, a mounting device has been proposed in Luxembourg Patent LU-87,010, filed on Oct. 6, 1987 (corresponding to U.S. Pat. No. 4,893,794), assigned to the assignee hereof, all of the contents of which are incorporated herein by reference, in the form of a cage which allows a clamp, of the same type as those described in Luxembourg Patents LU-83,917 and LU-87,546, to be fixed rigidly on a carriage supporting the working member on the support body. This cage blocks any rotation of the clamp and prevents the spindle from being subjected to a bending moment caused by offset forces. Furthermore, the device of patent LU-87,010 facilitates the assembling and disassembling of the clamp on the threaded spindle of the working member.
The ease with which the clamp is mounted is an important aspect as the clamp must be disassembled when the working member needs to be used for working with a drill bit, and the clamp must be remounted later if a piercing rod needs to be extracted from the tap hole using the same working member. Even with the mounting device of patent LU-87,010, the exchanging of the clamp for a drilling chuck and vice versa is extremely hard manual work and takes a lot of time which, in addition exposes workers to the risk of accidents.
It would therefore be desirable to provide a design for a very rigid universal chuck which would transmit hardly any of the offset forces to which it is subjected to the spindle and would allow a tensile/percussive force to be transmitted to the end of a piercing rod or transmit a substantial moment of rotation or force to a drill bit.
Such offset forces appear, in particular, when the piercing machine is withdrawn from the tap hole, while the rod is still not entirely free of the tap hole. It is often necessary to make such an early movement of the piercing machine away from its working position towards its withdrawn position. This is done in order to prevent the machine from being splashed by the jet of molten metal, which may occur when the tap hole is opened.
It should be noted that the chuck according to the present invention also overcomes a problem of the mounting device according to patent LU-87,010. This device blocks any rotation of the clamp although the working member can still be operated to produce a rotational moment. As a result, the spindle and certain elements of the mechanism of the working member are subjected to a maximum torsional force when the operator inadvertently initiates the rotational movement. This maximum torsional force is added to the normal stresses which occur when the rod is inserted and extracted, resulting in increased fatigue of the elements of the working member.